If your choices are from an enumeration: Create an extra row.Right click on that row. 'Edit condition for visible...' choose the enum value 'Other'.
Now that row is only visible if you choose 'Other' in the dropdown.
This is also possible with a boolean, that boolean should be something like 'ShowOtherTextBox'
If the dropdown list is an enumeration it can. You can then show a table row based on the enumeration value.